DOUBLE-LAYER POLYESTER FILM AND LAMINATED METAL SHEET

Description

Technical Field

[0001] The present disclosure relates to the field of film-laminated metal plates for metal packaging, and more particularly to a double-layer polyester film and a film-laminated metal plate.

Background Art

[0002] In the metal can making industry, a traditional practice is to coat the surface of a metal plate, and then shape the metal plate into a can. The solvent-based coating used in the coating process contains substances not safe to food, such as bisphenol A. During the coating process, a large amount of solvent evaporates, leading to environmental problems and large energy consumption. In recent years, the community of the industry has been committed to developing qualified new materials to address the above challenges.

[0003] Film-laminated steel is obtained by directly bonding a flexible polyester film to a metal plate together by hot melt lamination. It can meet the requirements of downstream users for can-making processing; differentiated printing; filling of various contents; acid, base or salt resistance in high-temperature sterilization; and long shelf life.

[0004] As environmental protection policies become more and more stringent and people's concern about food safety increases, the film-laminated steel has attracted more and more attention as a new environmentally friendly high-performance material in the canmaking industry. In recent years, the film-laminated steel products of some companies have been marketed. In order to meet the differentiated needs of different uses, the design of the raw materials and film structure of the polyester film needs to be optimized to meet the different requirements of the downstream processes.

[0005] CN 1839036 discloses a film-laminated metal plate and a drawn can using the same. This patent uses a double-layer film for lamination on a thin metal plate. Defects such as delamination and spalling occur easily during the canmaking process. A vinyl ionomer is used as an intermediate bonding layer which has poor performance during filling and high temperature sterilization.

[0006] CN 102432984 A discloses a cast polyester film and a metal plate and a metal can using the same. This patent uses a monolayer polyester film for lamination on a metal sheet. The insufficient barrier resistance of the film tends to provide the resulting can with poor resistance to acids, bases and salts in the filling process and poor performance in high-temperature sterilization.

[0007] CN 106142782 B discloses a polyester film for thermal lamination on steel and a method of preparing the same. This patent is characterized by lamination of a polyester film having a three-layer structure on a metal sheet. Because the performance of the film is limited by the high crystallinity of the lower skin layer, its processability and formability are not sufficient, and thus it is not suitable for uses involving deep drawing and complex deformation.

[0008] CN 102463725 A discloses a polyester film and a method of preparing the same. The polyester film of this patent has a three-layer (ABA) structure, and is used in the field of electronic tags. The film structure is characterized by the same resin material for two skin layers. Hence, it is difficult to meet the various performance requirements such as bonding with a metal plate, contact with a mold for molding, contact with the contents of a container, etc., which easily leads to difficulties in balancing performances, not suitable for the canmaking industry.

[0009] In the prior art, the single-layer film that meets the requirements of the deep drawing of a film-laminated metal plate does not have sufficient resistance to acids, bases or salts in the filling process or sufficient performance in high-temperature sterilization, while a multi-layer film that meets the requirements of resistance to acids, bases or salts in the filling process or sufficient performance in high-temperature sterilization cannot meet the requirements of the deep drawing process. For a traditional double-layer film, interlayer delamination of the film easily occurs during deep processing and forming. In a traditional film, an anti-blocking agent is added in the form of a silicon-containing chip master batch, such that there are too many types of resins in the film. Such a film is not suitable for deep drawing.

Summary

[0010] An object of the present disclosure is to provide a double-layer polyester film and a film-laminated metal plate, such that the requirements of deep drawing and complex deformation as well as the performance requirements of metal containers for food or beverage packaging can be satisfied.

[0011] In order to achieve the above object, the following technical solution is adopted according to the present disclosure.

[0012] According to one aspect of the present disclosure, there is provided a double-layer polyester film, wherein the double-layer polyester film comprises two layers including an upper layer and a lower layer, wherein the upper layer comprises a copolyester and an additional resin, and the lower layer comprises a copolyester and optionally an additional resin, wherein the copolyester comprises 800-2000 ppm by mass of SiO₂ added by in-situ polymerization. Preferably, SiO₂ in the copolyester has a content of 1000-1500 ppm by mass.

[0013] According to one aspect of the present disclosure, there is provided a double-layer polyester film, wherein the double-layer polyester film comprises two layers including an upper layer and a lower layer, wherein each of the upper layer and the lower layer comprises a copolyester and an additional resin, wherein the copolyester and the additional resin form a homogeneous mixture, wherein the copolyester comprises 1200 ppm by mass of SiO₂ added by in-situ polymerization.

[0014] In the double-layer polyester film according to one aspect of the present disclosure, the copolyester is a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol, and neopentyl glycol.

[0015] In the double-layer polyester film according to one aspect of the present disclosure, the copolyester has a melting point of 200-240°C. Preferably, the copolyester has a melting point of 210-230°C.

[0016] In the double-layer polyester according to one aspect of the present disclosure, the copolyester has an intrinsic viscosity of 0.68-0.72 dL/g.

[0017] In the double-layer polyester according to one aspect of the present disclosure, the copolyester has an intrinsic viscosity of 0.75-0.78dL/g after solid phase tackification.

[0018] In the double-layer polyester film according to one aspect of the present disclosure, the additional resin is one selected from the group consisting of homopolyester resins, polyester resins comprising a titanium dioxide master batch, and a mixture of a homopolyester resin and a golden polyester master batch resin.

[0019] In the double-layer polyester film according to one aspect of the present disclosure, the homopolyester resin is selected from the group consisting of PET, PBT and PPT. Preferably, the homopolyester resin has a melting point of 250-270 °C, preferably 258-265 °C.

[0020] In the double-layer polyester film according to one aspect of the present disclosure, the double-layer structured polyester film is made by co-extrusion and biaxial stretching at a temperature of 250-280°C.

[0021] In the double-layer polyester film according to one aspect of the present disclosure, the mass proportion of the additional resin in the upper layer is not more than 40%, and the mass proportion of the additional resin in the lower layer film is not more than 10%.

[0022] In the double-layer polyester film according to one aspect of the present disclosure, the mass proportion of the additional resin in the upper layer is 10-35% in some embodiments. Preferably, the proportion of the additional resin in the upper layer is not more than 30%. In some embodiments, the mass proportion of the additional resin in the lower layer is greater than 0% to ≤10%. In some embodiments, the lower layer does not comprise any additional resin.

[0023] In the double-layer polyester film according to one aspect of the present disclosure, the copolyester in the upper layer and the copolyester in the lower layer have the same melting point.

[0024] In a method of preparing the double-layer polyester film for medium-end to high-end food or beverage packaging according to one aspect of the present disclosure, the upper layer has a thickness of 8-25 µm, and the lower layer has a thickness of 3-6 µm.

[0025] Preferably, the lower layer has a thickness of 4-5 µm .

[0026] In the method of preparing the double-layer polyester film for medium-end to high-end food or beverage packaging according to one aspect of the present disclosure, the copolyester has a melting point of 200-240°C.

[0027] Preferably, the copolyester has a melting point of 210-230°C.

[0028] According to another aspect of the present disclosure, there is provided a film-laminated metal plate comprising a metal substrate and the abovementioned double-layer polyester film laminated on a surface of the metal substrate.

[0029] In the film-laminated metal plate according to another aspect of the present disclosure, the double-layer polyester film is thermally bonded to the surface of the metal substrate at a pressure of 2-10 Kg and a temperature of 180-260 °C.

[0030] In the film-laminated metal plate according to another aspect of the present disclosure, the metal substrate has a thickness of 0.10-0.50mm, and the metal substrate is selected from the group consisting of a chromium-plated steel plate, a tin-plated steel plate, a low-tin steel plate (a tin coating weight of ≤1.1g/m²), a galvanized steel plate, a cold-rolled steel plate, and a stainless steel plate.

[0031] According to still another aspect of the present disclosure, there is provided a metal container for medium-end to high-end food or beverage packaging, wherein the metal container is made of the film-laminated metal plate described above.

[0032] Compared with the prior art, the present disclosure has the following beneficial effects: the double-layer polyester film of the present disclosure exhibits an excellent performance in thermal bonding to the metal plate, and it's more special in that it can be used for deep drawing and various complex deformation processes without delamination in the film. In addition, the double-layer polyester film of the present disclosure provides better corrosion resistance than a monolayer film.

[0033] Due to the addition of SiO₂ to the polymer in the in-situ polymerization, the crystallization properties of the polyester film are improved uniformly on the whole. By substituting the traditional way of adding SiO₂ in the form of master batch, addition of a high melting point resin to the film is avoided. The above two points have improved the overall performance of the polyester film significantly, and the complex processing endurance and corrosion resistance of the film-laminated steel comprising the film of the present disclosure have been improved notably.

Detailed Description

[0034] The present disclosure provides a double-layer polyester film, wherein the double-layer polyester film comprises two layers including an upper layer and a lower layer, wherein the upper layer comprises a copolyester and an additional resin, and the lower layer comprises a copolyester and optionally an additional resin. Generally, the mass proportion of additional resin in the upper layer film is not more than 40%, for example, not more than 30%, or not more than 20%. In some embodiments, the mass proportion of the additional resin in the upper layer film is 10-30%. The mass proportion of additional resin in the lower layer film is not more than 10%, e.g., 0-10%. In some embodiments, the mass proportion of the additional resin in the lower layer film is greater than 0% to ≤10%, for example, 2-10%. The upper layer generally has a thickness of 8-25 µm, and the lower layer generally has a thickness of 3-6 µm. Preferably, the lower layer has a thickness of 4-5 µm.

[0035] The copolyester used to form the double-layer polyester film of the present disclosure is preferably a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol. Preferably, the copolyester comprises 800-2000 parts by mass of SiO₂ added by in-situ polymerization. Preferably, SiO₂ in the copolyester has a conent of 1000-1500 ppm by mass, preferably 1200 ppm by mass. The term "added by in-situ polymerization" or the like as used herein refers to mixing SiO₂ with the monomers for synthesizing the copolyester (i.e. terephthalic acid, ethylene glycol, isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl dimethanol), and then polymerizing to produce the copolyester according to the present disclosure. A conventional process for preparing PET polyester may be used to prepare the copolyester of the present disclosure. Preferably, the copolyester suitable for the present disclosure has a melting point of 200-240°C, preferably 210-230°C. Preferably, the copolyester of the present disclosure has an intrinsic viscosity of 0.68-0.72 dL/g. Preferably, the copolyester of the present disclosure has an intrinsic viscosity of 0.75-0.78 dL/g after solid phase tackification.

[0036] In the present disclosure, the intrinsic viscosity is measured by using a technique commonly used in the art.

[0037] As used herein, "additional resin" refers to a resin other than the copolyester described herein, including but not limited to a homopolyester resin, a polyester resin comprising a titanium dioxide master batch, a golden polyester master batch resin and any mixtures thereof.

[0038] Homopolyesters suitable for the present disclosure include but are not limited to PET, PBT and PPT. Preferably, the homopolyester resin has a melting point of 250-270 °C, preferably 258-265 °C.

[0039] In the polyester resin comprising a titanium dioxide master batch suitable for the present disclosure, the mass percentage of TiO₂ may be 40-80%, preferably 55-65%. Preferably, the polyester resin comprising a titanium dioxide master batch has a melting point of 250-270 °C, preferably 255-265 °C.

[0040] The golden polyester master batch resin preferably has a melting point of 250-270 °C, preferably 258-265 °C.

[0041] Preferably, when a mixture of two additional resins is used, such as a mixture of a homopolyester and a golden polyester master batch resin used as the additional resin, the mass ratio may be in the range of 1:3 to 3:1.

[0042] Preferably, when the lower layer comprises an additional resin, the additional resin is preferably a homopolymer resin.

[0043] The double-layer polyester film of the present disclosure can be made by co-extrusion and biaxial stretching at a temperature of 250-280°C.

[0044] The double-layer polyester film of the present disclosure can be used for medium-end to high-end food or beverage packaging.

[0045] There is further provided a film-laminated metal plate according to the present disclosure, wherein the film-laminated metal plate comprises a metal substrate and the double-layer polyester film described in any of the embodiments herein and laminated on a surface of the metal substrate. The double-layer polyester film of the present disclosure can be thermally bonded to the surface of the metal substrate at a pressure of 2-10 kg and a temperature of 180-260°C. Generally, the metal substrate has a thickness of 0.10-0.50 mm. The metal substrate suitable for the present disclosure may be selected from the group consisting of a chromium-plated steel plate, a tin-plated steel plate, a low-tin steel plate, a galvanized steel plate, a cold-rolled steel plate, and a stainless steel plate.

[0046] There is still further provided a metal container for medium-end to high-end food or beverage packaging according to the present disclosure, wherein the metal container is made of the film-laminated metal plate described in any of the embodiments in the present disclosure.

[0047] In the following detailed description, the objectives, features, and advantages of the present disclosure will become clearer and more apparent with reference to the nonlimiting examples, and the content is sufficient to enable those skilled in the art to understand and implement the present disclosure.

Example 1

[0048] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280°C, wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 200°C. The polyester film was manufactured by a double-layer co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 µm, and the lower layer had a thickness of 3-6 µm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 70% by mass of the copolyester having a melting point of 200°C and 30% by mass of a homopolyester having a melting point of 262°C; and the lower layer film comprised 95% by mass of the copolyester having a melting point of 200°C and 5% by mass of the homopolyester resin having a melting point of 262°C. The copolyester comprised 1200 ppm SiO₂ added by in-situ polymerization.

[0049] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 kg and a temperature of 180-260°C to obtain the film-laminated steel.

Example 2

[0050] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280°C, wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 240°C. The polyester film was manufactured by a double-layer co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 µm, and the lower layer had a thickness of 3-6 µm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 80% by mass of the copolyester having a melting point of 240°C and 20% by mass of a homopolyester having a melting point of 262°C; and the lower layer film comprised 90% by mass of the copolyester having a melting point of 240°C and 10% by mass of the homopolyester resin having a melting point of 262°C. The copolyester comprised 1200 ppm SiO₂ added by in-situ polymerization.

[0051] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 kg and a temperature of 180-260°C to obtain the film-laminated steel.

Example 3

[0052] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280°C.

[0053] The polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 230°C. The polyester film was manufactured by a double-layer co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 µm, and the lower layer had a thickness of 3-6 µm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 70% by mass of the copolyester having a melting point of 230°C and 30% by mass of an additional resin, wherein the additional resin comprised 1/3 by mass of a homopolyester having a melting point of 262°C and 2/3 by mass of a golden polyester master batch resin having a melting point of 262°C; and the lower layer film comprised 98% by mass of the copolyester having a melting point of 230°C and 2% by mass of the homopolyester resin having a melting point of 262°C. The copolyester comprised 1200 ppm SiO₂ added by in-situ polymerization.

[0054] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 kg and a temperature of 180-260°C to obtain the film-laminated steel.

Example 4

[0055] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280°C, wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 230°C. The polyester film was manufactured by a co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 µm, and the lower layer had a thickness of 3-6 µm. The upper layer comprised a mixture of the copolyester and the additional resin which were mixed homogeneously, wherein the upper layer film comprised 90% by mass of the copolyester having a melting point of 230°C and 10% by mass of a homopolyester having a melting point of 262°C; wherein the lower layer film comprised 100% by mass of the copolyester having a melting point of 230°C. The copolyester comprised 1200 ppm SiO₂ added by in-situ polymerization.

[0056] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 kg and a temperature of 180-260°C to obtain the film-laminated steel.

Example 5

[0057] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280°C, wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 230°C. The polyester film was manufactured by co-extrusion of an upper layer, an intermediate layer and a lower layer, wherein the total thickness of the upper layer and the intermediate layer was 8-25 µm, and the thickness of the lower layer was 3-6 µm. The upper layer and the intermediate layer each were a mixture of the copolyester and the additional resin which were mixed homogeneously. These two layers comprised completely the same resins, i.e., 90% by mass of the copolyester having a melting point of 230°C and 10% by mass of a homopolyester having a melting point of 262°C. These two layers were indeed a monolayer structure. The lower layer film comprised 100% by mass of the copolyester having a melting point of 230°C. The copolyester comprised 1200 ppm SiO₂ added by in-situ polymerization.

[0058] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 kg and a temperature of 180-260°C to obtain the film-laminated steel.

Example 6

[0059] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280°C, wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 230°C. The polyester film was manufactured by a double-layer co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 µm, and the lower layer had a thickness of 3-6 µm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 70% by mass of the copolyester having a melting point of 230°C and 30% by mass of a homopolyester having a melting point of 262°C; and the lower layer film comprised 90% by mass of the copolyester having a melting point of 230°C and 10% by mass of the homopolyester resin having a melting point of 262°C. The copolyester comprised 1200 ppm SiO₂ added by in-situ polymerization.

[0060] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 kg and a temperature of 180-260°C to obtain the film-laminated steel.

Example 7

[0061] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280°C, wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 210°C. The polyester film was manufactured by a co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 µm, and the lower layer had a thickness of 3-6 µm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 70% by mass of the copolyester having a melting point of 210°C and 30% by mass of a homopolyester having a melting point of 262°C; and the lower layer film comprised 92% by mass of the copolyester having a melting point of 210°C and 8% by mass of the homopolyester resin having a melting point of 262°C. The copolyester comprised 800 ppm SiO₂ added by in-situ polymerization.

[0062] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 kg and a temperature of 180-260°C to obtain the film-laminated steel.

Example 8

[0063] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280°C, wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 210°C. The polyester film was manufactured by a double-layer co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 µm, and the lower layer had a thickness of 3-6 µm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 68% by mass of the copolyester having a melting point of 210°C and 32% by mass of a homopolyester having a melting point of 262°C; and the lower layer film comprised 90% by mass of the copolyester having a melting point of 210°C and 10% by mass of the homopolyester resin having a melting point of 262°C. The copolyester comprised 2000 ppm SiO₂ added by in-situ polymerization.

[0064] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 kg and a temperature of 180-260°C to obtain the film-laminated steel.

Example 9

[0065] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280°C, wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 210°C. The polyester film was manufactured by a double-layer co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 µm, and the lower layer had a thickness of 3-6 µm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 65% by mass of the copolyester having a melting point of 210°C and 35% by mass of a homopolyester having a melting point of 262°C; and the lower layer film comprised 90% by mass of the copolyester having a melting point of 210°C and 10% by mass of the homopolyester resin having a melting point of 262°C. The copolyester comprised 1200 ppm SiO₂ added by in-situ polymerization.

[0066] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 kg and a temperature of 180-260°C to obtain the film-laminated steel.

Example 10

[0067] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280°C, wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 230°C. The polyester film was manufactured by a double-layer co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 µm, and the lower layer had a thickness of 3-6 µm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 65% by mass of the copolyester having a melting point of 230°C and 35% by mass of a homopolyester having a melting point of 262°C; and the lower layer film comprised 90% by mass of the copolyester having a melting point of 230°C and 10% by mass of the homopolyester resin having a melting point of 262°C. The copolyester comprised 1200 ppm SiO₂ added by in-situ polymerization.

[0068] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 kg and a temperature of 180-260°C to obtain the film-laminated steel.

Comparative Example 1

[0069] A monolayer polyester film was made from a modified PET resin having a melting point of 210°C by using a casting process.

[0070] Preparation of film-laminated steel: the monolayer polyester film prepared using the casting process was thermally bonded to the surface of a 0.19mm chromium-plated steel plate at a pressure of 2-10 kg and a temperature of 180-260°C to obtain the film-laminated steel.

Comparative Example 2

[0071] In a three-layer biaxially stretched polyester film, the upper layer resin was a 3µm PET resin, the intermediate layer comprised a 14µm blended resin of 265°C PET and 210°C PET (having a blending ratio of 7:3), and the lower layer comprised a 3µm modified PET resin having a melting point of 210°C.

[0072] Preparation of film-laminated steel: the prepared biaxially stretched polyester film was thermally bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 kg and a temperature of 180-260°C to obtain the film-laminated steel, wherein the lower layer was thermally laminated on the steel plate.

Comparative Example 3

[0073] In a three-layer biaxially stretched polyester film having an ABA structure, the upper layer resin was a modified PET resin having a melting point of 210°C, the intermediate layer comprised a PET resin having a melting point of 265°C, and the lower layer comprised a modified PET resin having a melting point of 210°C. The thickness ratio of the three layers was 1:8:1.

[0074] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 kg and a temperature of 180-260°C to obtain the film-laminated steel.

Comparative Example 4

[0075] A double-layer biaxially stretched polyester film was prepared using a vinyl ionomer as an intermediate bonding layer.

[0076] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 kg and a temperature of 180-260°C to obtain the film-laminated steel.

Test Example

[0077] The film-laminated metal plates obtained in Examples 1-10 and Comparative Examples 1-4 were processed using the Draw and Redraw process (DRD) under the following processing conditions. They were formed into can bodies by punching three times. The 20 µm functional film prepared was located on both the inner and outer sides of the cans at the same time.

Processing conditions (Draw and Redraw process)

1 .Blank diameter: 172mm.

2.First-pass processing conditions

Punch diameter: 114.5mm;

Die clearance: 0.36mm;

Blank holder force: 4000kg;

Mold assembly temperature before molding: 55°C.

3.Second-pass processing conditions

Punch diameter: 88mm;

Die clearance: 0.4mm;

Blank holder force: 3000kg;

Mold assembly temperature before molding: 55°C.

4.Third-pass processing conditions

Punch diameter: 65.3mm;

Die clearance: 0.43mm;

Blank holder force: 2000kg;

Mold assembly temperature before molding: 55°C.

After molding, conventional processes in can making were used for necking and flanging.

[0078] The cans prepared by the above methods were evaluated by the methods described below. The results are shown in Table 1.
(1) Adhesion of resin film in processing
The resin film layer laminated on the steel plate surface was visually evaluated to see whether it was peeled off or not during the processing steps of the DRD can prepared under the abovementioned forming and processing conditions. The result where no peeling occurred till the final step is excellent.
(2) Acid resistance performance: After the film-laminated steel was punched into a can (can size 691), acid resistance performance evaluation was performed to represent corrosion resistance performance evaluation. The film-laminated can was filled with a 20 g/L citric acid solution. After the can was capped, the solution was boiled at 121°C for 60 min. After cooling, the sample was taken out, and spots corroded by the acid on the surface of the sample were observed to evaluate the acid resistance performance of the film-laminated steel.
(3) Sulfur resistance performance: After the film-laminated steel was punched into a can (can size 691), sulfur resistance performance evaluation was performed to represent corrosion resistance performance evaluation. The film-laminated can was filled with a 1% Na₂S solution. After the can was capped, the solution was boiled at 121°C for 60 min. After cooling, the sample was taken out, and sulfide spots on the surface of the sample were observed to evaluate the sulfur resistance performance of the film-laminated steel.
(4) Flavor retention: After the film-laminated steel was punched into a can (can size 691), the can was filled with Tsingtao Snow Beer (manufactured by Tsingtao Brewery Company). After the filled can was sealed by using a conventional method, it was stored at 37°C for three weeks. The can was opened after the three weeks of storage, and an evaluation panel of 100 people inspected the taste retention of the content. The taste retention is excellent if 90 or more people considered that there was no difference in the taste of the content before and after the storage, good if 60 or more people considered that there was no difference, and poor if less than 60 people considered that there was no difference.

Table 1

Test Item	Adhesion in Processing	Acid Resistance	Sulfur Resistance	Flavor Retention
Example 1	⊚	⊚	⊚	Excellent
Example 2	⊚	⊚	⊚	Excellent
Example 3	⊚	⊚	⊚	Excellent
Example 4	⊚	⊚	○	Good
Example 5	⊚	⊚	○	Good
Example 6	⊚	⊚	⊚	Excellent
Example 7	⊚	⊚	⊚	Excellent
Example 8	⊚	⊚	⊚	Excellent
Example 9	○	⊚	⊚	Excellent
Example 10	○	⊚	⊚	Excellent
Comparative Example 1	⊚	○	Δ	Δ
Comparative Example 2	□	-	-	-
Comparative Example 3	□	-	-	-
Comparative Example 4	-	-	-	-

Note: in Table 1, □ means poor; Δ means mediocre; ○ means good; □ means very good; - means unable to be evaluated.

[0079] In the above Examples, the copolyester and an additional resin were blended and then formed into films having a double-layer structure including an upper layer and a lower layer. The formability of the films made by both the casting process and the biaxial stretching process is good, and the polyester films having a small thickness can be prepared. The polyester films have many advantages, such as excellent performance in thermal bonding to a metal plate, excellent endurance in deep drawing processing and complex deformation processing, and excellent corrosion resistance, widely useful in the metal packaging industry.

[0080] By adding SiO₂ to the copolyester in the in-situ synthesis, no conventional silicon-containing polyester chips are used in the polyester films, so involvement of a variety of polyester resins (having a melting point of greater than 245°C) is avoided. As a result, not only the basic requirements of an anti-blocking agent are satisfied, but also the overall performance of the films can be improved significantly.

[0081] Finally, it should be pointed out that although the present disclosure has been described with reference to the current specific examples, those skilled in the art should appreciate that the above examples are only used to illustrate the present disclosure, and are not used to limit the present disclosure. Various equivalent changes or substitutions can be made without departing from the concept of the present disclosure. Therefore, without departing from the essential spirit of the present disclosure, all changes and variations of the abovementioned examples will fall in the scope of the claims in the present disclosure.

Claims

1. A double-layer polyester film, wherein the double-layer polyester film comprises two layers including an upper layer and a lower layer, wherein the upper layer comprises a copolyester and an additional resin, and the lower layer comprises a copolyester and optionally an additional resin, wherein the copolyester comprises 800-2000 ppm by mass of SiO₂ added by in-situ polymerization.

2. The double-layer polyester film according to claim 1, wherein the double-layer polyester film comprises two layers including an upper layer and a lower layer, wherein each of the upper layer and the lower layer comprises a copolyester and an additional resin, wherein the copolyester and the additional resin form a homogeneous mixture, wherein the copolyester comprises 1200 ppm by mass of SiO₂ added by in-situ polymerization.

3. The double-layer polyester film according to claim 1, wherein the copolyester is a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol, and neopentyl glycol.

4. The double-layer polyester film according to claim 1, wherein the additional resin is selected from the group consisting of a homopolyester resin, a polyester resin comprising a titanium dioxide master batch, a golden polyester master batch resin and any mixtures thereof.

5. The double-layer polyester film according to claim 1, wherein the upper layer film comprises the additional resin in a proportion of not more than 40%, and the lower layer film comprises the additional resin in a proportion of not more than 10%.

6. The double-layer polyester film according to claim 1, wherein the upper layer film comprises the additional resin in a proportion of not more than 30%.

7. The double-layer polyester film according to claim 1, wherein the upper layer has a thickness of 8-25 µm, and the lower layer has a thickness of 3-6 µm.

8. The double-layer polyester film according to claim 7, wherein the lower layer has a thickness of 4-5 µm.

9. The double-layer polyester film according to claim 1, wherein the copolyester has a melting point of 200-240°C.

10. The double-layer polyester film according to claim 9, wherein the copolyester has a melting point of 210-230°C.

11. A method of manufacturing the double-layer polyester film according to claim 1, wherein the double-layer polyester film is made by co-extrusion and biaxial stretching at a temperature of 250-280°C.

12. A film-laminated metal plate comprising a metal substrate and the double-layer polyester film according to any one of claims 1-10 laminated on a surface of the metal substrate.

13. The film-laminated metal plate according to claim 12, wherein the metal substrate has a thickness of 0.10-0.50 mm, and the metal substrate is selected from the group consisting of a chromium-plated steel plate, a tin-plated steel plate, a low-tin steel plate, a galvanized steel plate, a cold-rolled steel plate and a stainless steel plate.

14. A method of manufacturing the film-laminated metal plate according to claim 12, wherein the method comprises direct lamination of the polyester film on a heated surface of the metal substrate by hot melt lamination.

15. A metal container for medium-end to high-end food or beverage packaging, wherein the metal container is made of the film-laminated metal plate according to any one of claims 12-13.